add a dev container definition for GitHub Codespaces / VS Code

.devcontainer/Dockerfile

@@ -0,0 +1,9 @@
+# See here for image contents: https://github.com/microsoft/vscode-dev-containers/tree/v0.205.2/containers/rust/.devcontainer/base.Dockerfile
+
+# [Choice] Debian OS version (use bullseye on local arm64/Apple Silicon): buster, bullseye
+ARG VARIANT="buster"
+FROM mcr.microsoft.com/vscode/devcontainers/rust:0-${VARIANT}
+
+# [Optional] Uncomment this section to install additional packages.
+# RUN apt-get update && export DEBIAN_FRONTEND=noninteractive \
+#     && apt-get -y install --no-install-recommends <your-package-list-here>